There is no single genetic mechanism behind sex differences in different species, and the existence of two sexes seems to have evolved multiple times independently in different evolutionary lineages. Other than the defining difference in the type of gamete produced, differences between males and females in one lineage cannot always be predicted by differences in another.

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Sex determination

The sex of a particular organism may be determined by a number of factors. These may be genetic or environmental, or may naturally change during the course of an organism's life. Although most species with male and female sexes have individuals that are either male or female, hermaphroditic animals have both male and female reproductive organs.

Environmental determination

In some species of reptiles, including alligators, sex is determined by the temperature at which the egg is incubated. Other species, such as some snails, practice sex change: adults start out male, then become female. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male.

In some arthropods, sex is determined by infection. Their sex is altered by Bacteria of the genus Wolbachia; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.

Mammalian female

The mammalian female is characterised by having two copies of the X chromosome as opposed to the male which carries only one X and one smaller Y chromosome. To compensate for the difference in size one of the female's X chromosomes is randomly inactivated in each cell.
Conversely in birds it is the female who is heterozygote and carries a Z and a W chromosome whilst the male carries two Z chromosomes.

The distinguishing characteristic of mammalian species is the presence of mammary glands on the female. The mammary glands are modified sweat glands that produce milk, which is used to feed the young during the period of time shortly after birth. Only mammals have the capacity to produce milk. The presence of mammary glands is most obvious on humans, due to the tendency of the female human body to store large amounts of fatty tissue near the nipples, resulting in prominent breasts. However, mammary glands are present in all mammals, although they are vestigial in male organisms.

Mammalian females are also unique in that they all bear live young (with the exception of monotremes, which lay eggs.) However, there are non-mammalian animals (such as sharks) whose eggs hatch inside their bodies, which gives the appearance that they bear live young.

Symbols

A common symbol used to represent the female gender is ♀ (Unicode: U+2640 Alt codes: Alt+12), a circle with a small cross underneath. This symbol also represents the planet Venus and is a stylized representation of the goddess Venus' hand mirror.

Etymology

The word female comes from the Latin femella, the diminuative form of femina, meaning 'woman', which is not actually related to the word 'male.' The word was probably originally femella, meaning "young girl". In the late 14th century, the English spelling was altered so that the word paralleled the spelling of "male".

Sources

Ayers, Donald M. English Words from Latin and Greek Elements. Second Edition. 1986. University of Arizona Press. United States.